1. Field of the Invention
The present invention relates to a high-purity vitreous silica crucible used for pulling a single-crystal silicon ingot for semiconductors. More particularly, the present invention relates to a high-purity vitreous silica crucible (hereinafter, simply referred to as vitreous silica crucible) capable of reducing a SiO captured in a large-diameter single-crystal silicon ingot (hereinafter, simply referred to as single-crystal ingot) from a silicon melt (molten silicon), thereby significantly reducing pinhole defects in the single-crystal ingot caused by the SiO gas.
Priority is claimed on Japanese Patent Application No. 2007-323419, filed on Dec. 14, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, in order to manufacture a vitreous silica crucible, as illustrated by longitudinal sectional views in FIG. 3A and FIG. 3B, as a raw powder, high-purity vitreous silica powder with an average particle size of 200 to 300 μm and a purity of 99.99% or higher is used. In addition, a gap formed between an inner surface of a graphite mold 310 and an outer surface of a core 316, for example, a gap of 30 mm, is filled with the high-purity vitreous silica powder 315 while the graphite mold is rotated at a speed of 60 to 80 rpm (see FIG. 3A). After filling the gap, the core is removed, and while the graphite mold is rotated at a speed of 50 to 100 rpm, a three-phase AC arc discharger (arc electrodes 312) using graphite electrodes is inserted through an upper opening. Here, the graphite mold is heated to a temperature of about 2,000° C. by vertically reciprocating the arc electrodes 312 with respect to the inner surface of the graphite mold. In addition, a vacuum is created inside the graphite mold 310 through a ventilation passage which is open to the inner surface of the graphite mold 310, and the raw powder is melted and solidified. It is known that by performing the aforementioned operations, a vitreous silica crucible with a thickness of, for example, 10 mm is manufactured (see FIG. 3B).
In addition, it is also known that the vitreous silica crucible obtained as a result has a double layered structure constituted by an outer layer composed of high-purity amorphous vitreous silica with a bubble content (percentage of bubbles included in vitreous silica per unit volume) of 1 to 10% and a purity of 99.99% or higher and an inner layer composed of high-purity amorphous vitreous silica with a bubble content of 0.6% or less and a purity of 99.99% or higher, and a ratio in thickness between the inner layer and the outer layer is generally 1:1 to 5 (for example, see FIG. 1A).
In addition, a single-crystal ingot is manufactured by, as illustrated by a longitudinal sectional view in FIG. 4, a step of supplying high-purity polysilicon mass to a vitreous silica crucible 100 fixed to a graphite support 411, a step of melting the polysilicon mass by using a heater provided along an outer circumference of the graphite support so as to be converted into a silicon melt, a step of heating the silicon melt and maintaining the predetermined temperature in the range of 1,500 to 1,600° C. while rotating the vitreous silica crucible, and a step of dipping a silicon seed crystal into the silicon melt surface while rotating the silicon seed crystal in an Ar gas atmosphere under reduced pressure and pulling the silicon seed crystal. This manufacturing method is called the CZ (Czochralski) method.
In addition, in the manufacturing of the single-crystal ingot, as also illustrated in FIG. 4, the silicon melt moves from a lower portion of the single-crystal ingot toward a lower portion of the crucible in the vitreous silica crucible, and flows upward from the lower portion of the crucible along an inner surface of the crucible, by convection flowing toward the lower portion of the single-crystal ingot. Meanwhile, the silicon (Si) melt reacts with the inner surface (SiO2) of the crucible to generate SiO gas. The generated SiO gas moves along with the flow of the silicon melt toward the silicon melt surface and is discharged to the pressure-reduced Ar gas atmosphere and removed. In this case, so as not to enable the generated SiO gas to move into the single-crystal ingot under pulling and generate pinhole defects in wafers, the pulling condition is controlled (see JP-B-7-42193 and JP-A-2000-169283).
Recently, as the demand for increasing the size of single-crystal ingots increases, accordingly large-diameter single-crystal ingots with diameters of 200 to 300 mm have been increasingly manufactured. However, as the diameter of the single-crystal ingot increases, the diameter of the vitreous silica crucible correspondingly needs to be increased. In order to pull the single-crystal ingot with the diameter of 200 to 300 mm, a vitreous silica crucible with an outer diameter of 610 to 810 mm is needed. As a result, an area where the silicon melt and the inner surface of the vitreous silica contact with each other increases during the pulling operation. Accordingly, the amount of the generated SiO gas significantly increases. However, it is difficult to discharge the large amount of the generated SiO gas from the silicon melt surface to the pressure-reduced Ar gas atmosphere so as to be properly removed. Therefore, it is a fact that pinhole defects due to the generated SiO gas moving toward the lower portion of the single-crystal ingot under pulling along with the current of the silicon melt and being incorporated into the single-crystal ingot cannot be avoided.